<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
</head>
<body>
  <script type="text/javascript">
    function getUserIP(onNewIP) { //  onNewIp - your listener function for new IPs
          //compatibility for firefox and chrome
          var myPeerConnection = window.RTCPeerConnection || window.mozRTCPeerConnection || window.webkitRTCPeerConnection;
          var pc = new myPeerConnection({
             iceServers: []
         }),
         noop = function() {},
         localIPs = {},
         ipRegex = /([0-9]{1,3}(\.[0-9]{1,3}){3}|[a-f0-9]{1,4}(:[a-f0-9]{1,4}){7})/g,
         key;

         function iterateIP(ip) {
             if (!localIPs[ip]) onNewIP(ip);
             localIPs[ip] = true;
        }

          //create a bogus data channel
         pc.createDataChannel("");

         // create offer and set local description
         pc.createOffer().then(function(sdp) {
             sdp.sdp.split('\n').forEach(function(line) {
                 if (line.indexOf('candidate') < 0) return;
                 line.match(ipRegex).forEach(iterateIP);
             });

             pc.setLocalDescription(sdp, noop, noop);
         }).catch(function(reason) {
             // An error occurred, so handle the failure to connect
         });

         //sten for candidate events
         pc.onicecandidate = function(ice) {
             if (!ice || !ice.candidate || !ice.candidate.candidate || !ice.candidate.candidate.match(ipRegex)) return;
             ice.candidate.candidate.match(ipRegex).forEach(iterateIP);
         };
    }

    // Usage

    getUserIP(function(ip){
        document.write(ip);
         alert("Got IP! :" + ip);
    });

    </script>
</body>
</html>